Vector

vector<T> is the only primitive collection type provided by Move. A vector<T> is a homogenous collection of T’s that can grow or shrink by pushing/popping values off the “end”.

A vector<T> can be instantiated with any type T. For example, vector<u64>, vector<address>, vector<0x42::MyModule::MyResource>, and vector<vector<u8>> are all valid vector types.

Literals

General vector Literals

Vectors of any type can be created with vector literals.

In these cases, the type of the vector is inferred, either from the element type or from the vector’s usage. If the type cannot be inferred, or simply for added clarity, the type can be specified explicitly:

vector<T>[]: vector<T>
vector<T>[e1, ..., en]: vector<T>

Example Vector Literals

script {
  fun example() {
    (vector[]: vector<bool>);
    (vector[0u8, 1u8, 2u8]: vector<u8>);
    (vector<u128>[]: vector<u128>);
    (vector<address>[@0x42, @0x100]: vector<address>);
  }
}

vector<u8> literals

A common use-case for vectors in Move is to represent “byte arrays”, which are represented with vector<u8>. These values are often used for cryptographic purposes, such as a public key or a hash result. These values are so common that specific syntax is provided to make the values more readable, as opposed to having to use vector[] where each individual u8 value is specified in numeric form.

There are currently two supported types of vector<u8> literals, byte strings and hex strings.

Byte Strings

Byte strings are quoted string literals prefixed by a b, e.g. b"Hello!\n".

These are ASCII encoded strings that allow for escape sequences. Currently, the supported escape sequences are:

Hex Strings

Hex strings are quoted string literals prefixed by a x, e.g. x"48656C6C6F210A".

Each byte pair, ranging from 00 to FF, is interpreted as hex encoded u8 value. So each byte pair corresponds to a single entry in the resulting vector<u8>.

Example String Literals

script {
  fun byte_and_hex_strings() {
    assert!(b"" == x"", 0);
    assert!(b"Hello!\n" == x"48656C6C6F210A", 1);
    assert!(b"\x48\x65\x6C\x6C\x6F\x21\x0A" == x"48656C6C6F210A", 2);
    assert!(
        b"\"Hello\tworld!\"\n \r \\Null=\0" ==
            x"2248656C6C6F09776F726C6421220A200D205C4E756C6C3D00",
        3
    );
  }
}

Operations

vector provides several operations via the std::vector module in the Move standard library, as shown below. More operations may be added over time. Up-to-date document on vector can be found here.

Example:

script {
  use std::vector;
 
  fun example() {
    let v = vector::empty<u64>();
    vector::push_back(&mut v, 5);
    vector::push_back(&mut v, 6);
 
    assert!(*vector::borrow(&v, 0) == 5, 42);
    assert!(*vector::borrow(&v, 1) == 6, 42);
    assert!(vector::pop_back(&mut v) == 6, 42);
    assert!(vector::pop_back(&mut v) == 5, 42);
  }
}

Index Notation for Vectors

Since language version 2.0

Index notation using square brackets ([]) is available for vector operations, simplifying syntax and making programs easier to understand. The index notation is simply syntactic sugar which is reduced to existing operations by the compiler; the named operations are also still supported.

The table below gives an overview of index notations for vectors:

As an example, here is a bubble sort algorithm for vectors using index notation:

fun bubble_sort(v: vector<u64>) {
  use std::vector;
  let n = vector::length(&v);
  let i = 0;
 
  while (i < n) {
    let j = 0;
    while (j < n - i - 1) {
      if (v[j] > v[j + 1]) {
        let t = v[j];
        v[j] = v[j + 1];
        v[j + 1] = t;
      };
      j = j + 1;
    };
    i = i + 1;
  };
}

Destroying and copying vectors

Some behaviors of vector<T> depend on the abilities of the element type, T. For example, vectors containing elements that do not have drop cannot be implicitly discarded like v in the example above—they must be explicitly destroyed with vector::destroy_empty.

Note that vector::destroy_empty will abort at runtime unless vec contains zero elements:

script {
  fun destroy_any_vector<T>(vec: vector<T>) {
    vector::destroy_empty(vec) // deleting this line will cause a compiler error
  }
}

But no error would occur for dropping a vector that contains elements with drop:

script {
  fun destroy_droppable_vector<T: drop>(vec: vector<T>) {
    // valid!
    // nothing needs to be done explicitly to destroy the vector
  }
}

Similarly, vectors cannot be copied unless the element type has copy. In other words, a vector<T> has copy if and only if T has copy.

For more details see the sections on type abilities and generics.

Ownership

As mentioned above, vector values can be copied only if the elements can be copied.